Advice: final plans for 38X20 house plans
I have been reading for many months on this site and finally going forward with our build in the coming weeks. I would like to run my plans by you guys for any tips and or advice.
Zone 6: Maine
Size: 38X20 two story (second is 3′ stem wall, no dormers)
Slab: We are planning on running two layers of ICF blocks on a footing insulated out by 3′-4′ foot under the footing for frost protection. Once the first story is built, we plan on pouring the slab internal of the building when its heated. Under slab will be 10mil barrier and 3″-4.5″ layers of comfortboard 80, or EPS if I can get it in time. Does the stem wall need to be tied in with rebar to the slab?
Walls: 2X6 construction, 24′ OC filled with Rockwool and seethed with Zip R-12. My concern is how close the R12 is to the limits of 35% external insulation, is this an issue?
Roof: unknown at the moment, possible, 2X12 rafters, 24″ OC with standing seam roofing.
Heatings: 4 zone mini splits, one in living, bath and each bedroom. With a wood stove as primary.
Thanks everyone!
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Replies
Aja,
Have you had a structural engineer stamp the framing plans? I ask because one of my past homes had a second story that originally matched your description of a 3 foot wall that formed the seat for the roof rafters. I discovered some disturbing evidence of the roof spreading over the 50-60 years before we bought the money pit. It is important to note that the framing in this case was balloon style for the second floor which could well have contributed to the elements failure.
While it was originally framed for two rooms, one each side of the center staircase, a lift dormer was executed later to create a bath and orphaned bedroom (access only through another room). When we overhauled the bath I could see that the 2x4 parts of short wall section had all developed splits. The 2x4's on the non dormer side also showed signs of splitting due to outward pressure from the roof rafters spreading outward.
In fairness to the house, it was still standing at the 80 plus mark when we left, but I have since heard of a "rule of thumb" regarding the height of the cross tie members for higher pitched roofs. That rule being, set the cross ties no higher than about the 1/3 the height from peak to top plate. The three foot wall is a useful way to "cheat in" an acceptable amount of head and floor room without going to a dormer or gable. It does leave for some funky closet space, which we are glad to be done with.
I am guessing that your framing will be modern platform framing for both floors which causes me to wonder if the potential for wind forces against the roof and possible spreading of the rafter legs below the collar ties will be an issue. Going to 24" on center will demand more of each rafter and most likely up the required sheathing thickness for racking and snow load reasons.
I am first to say that I do not have standing as a structural advisor, so if all that has been checked out good. I would then ask, how do you plan to insulate the second floor? A really good option in my opinion would be to go unvented with sufficient exterior insulation to make every inch of the second story conditoned. I did that with 8 1/4" nailbase over the roof sheathing/WRB and put R19 batts in the rafter bays where I used 2x12's instead of trusses.
If you are planning on venting you will forced to give up a significant fraction of insulation room and most likely need to cobble on solutions for the depth needed to make code for R value. Having a three foot wall instead of a pie shape to deal with might make the process a bit less miserable, but as you have already planned for ZipR and batts to get good results below, why not follow suit and do the roof in a similar way. It will make a lot of decisions about finishing in the upstairs much simpler.
The R12 Zip has been discussed on GBA for shear strength questions. At least one contributor did not have a good experience due to inept installation. Do you really have a good rapport with the builder or are you doing this all on your own? I think the relative R values are acceptable due to the fact that the Whole Wall R value of the framed and batted part of the wall will come in a bit less due to the frame losses. Others will surely weigh in an give you solid advice on how close you are cutting it.
FWIW, I did not pin any of my basement slabs, (three levels) to the exterior walls and have had no issues. I am over washed rock all the way to the bedrock I built on so that may be a big factor. The 3" reclaimed XPS has been great, but I wish I had gone to 4" under the slab and outside the foundation walls. I am unclear on your description of a footing on 3-4' of foam, but I am guessing that you mean out from the footing to create a frost protected foundation. Again, I will defer to higher level GBA members and only say that your soil engineer or inspector will have something to say about it. I have seen similar done here in CZ6a-b despite the local frost point being declared 48". Wet ground will be the big question to think about.
Best of luck as you move forward. Planning now is much better than patching later.
Even the smallest 4 zone multi split would be at least 3x the heat load of the house. Usually best option is for a wall/floor mount on the main floor and a ducted units for all the bedrooms.
Skip the story and a half and build the full 2 story the effort you will spend try unsuccessfully to keep the warm and cold air separated and insulating the sloped ceilings is better spent a simple 2 story that 25% bigger and 100% better.
Walta